In 1493, the year after Columbus sailed west and found “the Indies,” Pope Alexander VI (the notorious Borgia Pope) issued the Bulls of Donation dividing the New World between Spain and Portugal. The problem was that Portugal felt the Papal grant gave it too little land and prevented their hoped for conquest of India. So the Iberian states negotiated an amendment to the Papal grant with the Treaty of Tordesillas the following year. The treaty shifted the Papal line west, allowing Portugal to claim lands in South America that eventually became Brazil.
The concept seems audacious. Two European powers and the Pope were carving up lands they had never seen. But there was some logic to the madness. Portugal had been aggressively exploring the coast of Africa and in 1498 would discover the direct sea route to India. After Columbus discovered the Americas, Spain was the only other power at the time with the interest and the means to go exploring.
The Papal grant and Tordesillas suffered from many problems. The first practical one was it did not acknowledge that the Earth is a sphere. Giving Spain lands to the west of the line and Portugal lands to the East became a problem as both powers raced to discover and claim their lands first. When Portugal discovered the Moluccas in 1512 and Magellan visited them in his voyage in 1521, it prompted Spain to argue that the dividing line created two hemispheres (and the Moluccas were of course in their hemisphere). The two powers would resolve this by the Treaty of Zaragoza in 1529 by drawing a line on the other side of the world.
But by then there were other problems with Tordesillas. As other European powers realized just how large the world was, they had no intention of letting the Pope hand it over to the Iberians – Catholic loyalties went only so far. “Show me Adam’s will!” was the mocking response of Francis I of France to the Papal edict. It got worse as England and the Netherlands turned Protestant during the Reformation. Papal grants of land had meaning for these powers any more.
The line itself was meaningless between 1580 and 1640 when Spain and Portugal were united under one crown. By the late 17th century both powers had been relegated to second tier and the treaty was meaningless as the French and British clashed in North America and India while the Dutch expelled the Portuguese from Indonesia, Ceylon and South Africa. Even though the Treaty faded into irrelevance it has still been used by Chile to support its claims to parts of Antarctica and Argentina for the Falklands.